Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dennyg on January 09, 2018, 04:34:15 pm
-
I use a simple DC heater supply for the pre-amp tubes in 3 builds - 3A bridge plus a 10,000uf cap; neg side is elevated about 40V in all three.
In one build, the AC feeds power tubes then feeds the DC circuit, with an expected slight increase (from 6.3AC to 6.5DC) due to rectification.
However in the other two builds, I put the DC circuit before the power tubes - i.e. AC feeds the bridge then branches off to feed the power tubes. The DC actually drops (6.3AC to 6.1DC) in both amps. PT has 5A heater supply feeding 2 6V6's and 4 12AX7's - current is measured at 2.4A
Explanation por favor?
-
what is it unloaded?
-
AC 6.6, DC 8.1 unloaded
-
I don't see a reason one is dif from the other, try temping in another 10k uF :dontknow:
-
thanks man - i'm not trying to remedy this as the voltages are fine and the DC is doing it's job eliminating heater hum - i'm just curious as to why i'm getting a DC voltage drop when placing the bridge before the PA tubes vs after. There is a benefit in that I don't have to use a dropping resistor before the bridge to reduce the rectified voltage as I did on my first build using DC.
-
The rectifier and cap are saggy. The extra load (more than double) sags more.
Why do you think you need DC on the 6V6es? Signal levels here are huge. Hum should not be an issue.
-
only using DC on the pre-amp PRR - confusion is that i get a lower rectified DC voltage (6.1V) when the bridge is fed AC before feeding the 6V6's vs when bridge sits after the power tubes (6.5V) - AC is 6.3 in both cases. One thought - does the bridge look like a series circuit to the AC circuit when spliced in before the 6V6's?
-
The bridge is directly across (parallel) the 6.3VAC filament string in either case. All you have done is moved it a little farther down the wire. The voltage should be exactly the same. Maybe you just have a poor connection somewhere in the AC string.
-
> confusion is that i get a lower rectified DC voltage (6.1V) when the bridge is fed AC before feeding the 6V6's vs when bridge sits after the power tubes (6.5V) - AC is 6.3 in both cases.
Before, after?
Electric charge fills ALL the paths it can find, at the same time.
IF I understand the situation, it is like this? You moved a 0.9A load over to the DC side?
-
If so, imagine these are sprinkler heads, the rectifier is some kind of restricty valve. Moving two fat heads from the supply side over after the valve is sure to give lower pressure beyond the valve.
-
Also note how ugly the wave form is, uglier with the higher load. You have nearly 2Vpp, say 0.6Vrms, of 120Hz and higher harmonics, instead of 6V of fairly pure 60Hz. I would expect that much ripple to make much more buzz offense than a pure AC even of larger amplitude, because ears and g-speakers work so much less in deep bass.
-
IF I understand the situation, it is like this? You moved a 0.9A load over to the DC side?
I don't think that's what he's saying. I think he's saying he picked off the AC before feeding the 6V6s. Then he picked off the same AC after the 6V6s. I think the DC circuit is powering the same load in either case.
-
I don't think that's what he's saying. I think he's saying he picked off the AC before feeding the 6V6s. Then he picked off the same AC after the 6V6s. I think the DC circuit is powering the same load in either case.
Yes that's what I'm doing. I would've thought I had weird connection or something except that I have this same circuit running in two amps with 6V6's and both yield the same voltages.
-
The three circuits are electrically identical. Make the circuits mechanically identical.
However, 6.1v or 6.5v are both totally acceptable voltages for heating those tubes. Not an issue IMO.
-
Two of the circuits are mechanically identical yielding same voltages, the third places the bridge/DC after the power tubes.
Actually the placement ahead of the power tubes yields a net advantage to the third circuit in that it drops the voltage for DC - on the 3rd circuit I added an R22 dropping to produce lower voltages (and stress on the heaters), which I can otherwise eliminate. Much of the joy of this hobby is derived from learning the theory - producing a quiet, great sounding amp is equal to understanding the electronics behind it.
BTW, I'm almost done with a 20W version of your Dual 50 (one of the two DC amps referenced above). I finished the JCM800 circuit last night and what a glorious chain-saw 800 tone! I did make a couple of mods that Freedman did - used a 220K Ra on first stage, and 33K (he used 39K) Rk for stage 2 - gooses the crunchy gain a bit. I should have the plexi circuit wired up tonight. Made a few more mods that I'll include in a build thread shortly. I hope to be the first to build this amp using a term strip / gothic ring layout and share with others so inclined.
-
...I'm almost done with a 20W version of your Dual 50 ...using a term strip / gothic ring layout and share with others so inclined.
I'd really like to see that project! I bet others will too. Please start a thread and post some pretty pics.
-
I have an obligation to bring this thread to closure for those of you either contributed or for any poor soul stumbling upon this in the future and scratching their head. I'm embarrassed to state that the circuits are not mechanically identical, but the two builds displaying lower voltages in fact have 2 more pre-amp tubes than the other, hence explaining the voltage difference. It's one of those cases where the observer became so focused/obsessed on the perceived difference (in this case placement of the bridge) that he failed to consider the other variables. In summary, the voltage difference had nothing to do with placement of the bridge. I'll now don a hair-shirt for the weekend to pay for my sins.
-
You a bad man. A very bad man. I send you to the corn field! :icon_biggrin: